This invention relates to a method and apparatus for automatically monitoring the density and viscosity of a liquid, such as drilling mud.
Drilling fluids used to drill wells into the earth are commonly called drilling muds because the original drilling fluid was simply water that mixed with clays in the earth to produce a thin natural mud. Typically, drilling mud is pumped down the drill string, through nozzles in the end of the bit and then upwardly in the annulus between the drill string and the wall of the bore hole. Drilling mud has a variety of functions and must accordingly have comparable capabilities. Cuttings generated by the bit are moved away from the bottom of the hole and then upwardly through the annulus to the surface to present a clean rock face to be drilled. The bit is cooled and lubricated by the drilling mud. The mud also forms a wall cake on the exposed face of the well bore to prevent the drilled formations caving into the bore hole. The pressure of fluids in the formations penetrated by the bit is counterbalanced, or at least partially so, by the hydrostatic weight of the mud column in the hole. The drilling mud is modified to prevent undue effects on the bore hole wall, e.g. to prevent shale swelling. In water based muds, materials are added to prevent undue water loss into permeable formations penetrated by the bit. Various materials are added to reduce friction between the drill string and the bore hole wall. An almost endless list of substances have been added to drilling mud for a variety of reasons.
Two important characteristics of drilling mud are mud weight and viscosity. Mud weight is important to counterbalance the pressure in permeable formations penetrated so the well does not blow out. In English measurement systems, mud weight is reported in pounds per gallon. Most wells are drilled overbalanced, i.e. the mud weight is sufficient to contain formation pressures. Some wells are drilled underbalanced, i.e. the mud weight is not sufficient to wholly contain formation pressures, so the contents of drilled formations flow into the bore hole and are circulated to the surface. When it is desired to drill overbalanced, mud weights that are too low cause a well to kick or blow out. When it is desired to drill overbalanced, mud weights that are too high normally produce only unnecessary costs although there is a slight danger of causing formations up the hole to break down and take mud. When it is desired to drill underbalanced, mud weights that are too low create an excessive pressure differential across the formation face. When it is desired to drill underbalanced, mud weights that are too high may result in drilling the well overbalanced. In any event, abrupt changes in mud weight are a reliable signal that something is amiss and, in some situations, is a sign that disaster is approaching.
Mud weight is conventionally measured with a beam balance having a small metal cup at one end receiving a fixed amount of mud and a sliding weight on a lever arm fixed to the cup. The beam is placed on a pivot and the sliding weight moved along the lever arm until it balances. The density of the mud is read off the lever arm adjacent the slide. Mud weight is controlled by the addition of weight materials to the mud, usually barite which is a naturally occurring barium sulfate or hematite which is an iron oxide.
Viscosity of drilling mud is important because it is a measure of the capacity of the mud to move cuttings up the hole and a measure of the gel strength of the drilling mud which is related to the thixotropic capacity of the mud, i.e. the ability to set up as a gel or semi-solid thereby suspending cuttings to prevent them from settling to the bottom of the bore hole when the mud is quiescent. Viscosity is conventionally measured by adding a predetermined quantity of mud to a funnel of predetermined shape, known as a viscosimeter, Saybolt funnel, or viscosity funnel, allowing the predetermined volume to run out of the funnel, and measuring the time for the funnel to empty. Viscosity of drilling mud is typically measured in seconds. With low cost, water based muds, viscosity is controlled by the addition of bentonite which is often called gel. Bentonite is a naturally occurring swellable clay and has been used for decades as the standard viscosifier in drilling muds. Many other materials, such as polymers, are also commonly used.
Drilling mud has many other properties that are measured by a technician known as a mud man. These properties include water loss, pH, gel strength, and the like. Although these properties are of importance for a variety of different reasons, to the drilling contractor or person responsible for drilling the well and delivering a logable hole at the least cost, the most important mud characteristics are mud weight and viscosity.
It is known in the art to automatically monitor mud weight and/or viscosity of drilling mud as shown in U.S. Pat. Nos. 2,132,015; 2,252,014; 3,074,266 and 5,052,219.
In this invention, an automated device is provided to periodically measure the density and viscosity of any suitable liquid, such as solutions, slurries, or suspensions of any type, for example drilling mud, paint, and the like. Although the method and apparatus of this invention are applicable to other liquids, this invention is described in conjunction with drilling mud because that is a particular niche for which the invention has application.
A suitable print out is provided, preferably at a remote location, such as the driller""s station, a central office, or a location handy to a drilling consultant. An important feature of this invention is using a single container to weigh a quantity of mud and measure the time it takes for the mud to drain out of the container thereby providing a measure of viscosity.
A variety of features allow the device of this invention to produce consistently reliable results: (1) the measuring container is washed at the end of every measurement cycle and, if the weight of the container does not fall to a predetermined empty weight, the container is rewashed; (2) if rewashing does not reduce the weight of the container to its empty weight, the conclusion is that mud solids have adhered to the container which can corrupt subsequent measurements and an alarm is accordingly sounded and the device turned off; (3) an alarm is sounded if the mud weight measures a value which is too low and/or too high; (4) the too low weight and/or the too high weight limits can be set by an operator; (5) in the event the measuring container overflows, the device is shut off and an alarm sounded; (6) when the supply valve to the mud container opens, the weight of the container is monitored so that, if the weight of the container does not increase, the conclusion is made that something is amiss with the supply valve and/or there is a blockage in the mud line; (7) multiple weight measurements are taken and then averaged to provide the reported mud weight; and (8) a relatively large volume of mud is weighed, as compared to the conventional beam balance thereby providing greater accuracy because small errors are not magnified by the multiplication that necessarily goes on to convert the measured value to pounds per gallon.
It is an object of this invention to provide an improved method and apparatus for automatically monitoring drilling mud.
Another object of this invention is to provide a method and apparatus for measuring the density and viscosity of drilling mud.
A further object of this invention to provide apparatus for monitoring drilling mud which is inexpensive and reliable and which produces consistent results.
Another object of this invention is to provide an apparatus for measuring drilling mud weight and viscosity which employs a single measuring container.
These and other objects and advantages of this invention will become more fully apparent as this description proceeds, reference being made to the accompanying drawings and appended claims.